I have been working on a miniature, low power wireless remote node based on the ATmega32U4 MCU (the same used in Leonardo Arduino). This is a Open Source Project, all design files and documentation is available here: http://harizanov.com/wiki/wiki-home/funky-sensor-v2/.I wish to say that I am amateur, only started learning electronics on hobby level recently.

The highlights are:

2*2 cm in size, only few mm thick

can be powered from a CR2032 coin cell

0.04mA @ 3V in sleep mode

Programmed via the USB - just like Arduino Leonardo

Has a RFM12B radio module for wireless communication with other sensor nodes or another node acting as a gateway (logging the data or forwarding it to the Internet)

Option for on-board voltage step-up boost regulator

I build these by hand using hot air gun - a slow process, but the outcome is beautiful:

Nice work. I'm wanting to do something similar with the mini-pro... where I put in pull down resistors on 6 of my digital inputs.... and then voltage dividing resistors for 4 or 7 (application dependent) 3mm LEDs. Of course I can punt the resistors if I go with the 8mhz 3.3 variety.

I want to permanently mount an RJ45 header for the inputs (I'll run power, 6 inputs and a spare down a section of Cat5e to my devices in the field) and a 9v power header for power. The inputs would of course also need a pull down resistor to ground.

Then I 'd need 10 additional pins that I just connect a ribbon cable to, in order to accommodate the interface elements mounted on a small board on the enclosure (buttons, LEDs, etc...).